Match the first instance of a string to the left of a given cell

I have set up a row whose cells can contain one of three values, “A”, “Z” or “”. I want to find the index of the first cell to the left of a given cell.



Cell C15 is the cell I am referencing from.
Cell C11 is the cell I am looking for the index to.
MATCH seems to always go back to C1. I want to find the index to C11.

Please upload a real sample file (.ods file) here.

Basically MATCH and LOOKUP functions are made for the kind of tasks, but all these standartd means search top down or left to right. Everything you can do for matching in revesre direction is a workaround, at best a slightly efficient one.
Therefore: If you design a sheet where matching may be needed some day, try to organize it in a way fit for the application of standard functions.